A36 (Grade 50) versus A572 Gr 50

[BloEngineer]

I am about to finalize drawings for a project where I have specified the use of ASTM A572 Grade 50 rods. The rods are partially threaded and epoxy anchored into an existing concrete structure and has a smooth, greased body to be embedded in new concrete. The purpose of the rod is to provide fixity to the existing concrete and an essential slip connection to the new concrete, so that it acts as a type of sliding connection. The rods have been sized accordingly to resist the applied shear based on the strength of the steel, which governs over the bond of the epoxy.

I selected ASTM A572 Grade 50 rods because of the tensile strength of 65 ksi, which meets my shear resistance requirements. The project is Design-Build, and the contractor is requesting the use of ASTM A36, Grade 50 rods, which I'm assuming are more readily available (thus cheaper). In looking at the AISC Manual, the tensile strength for A36 steel varies from 58-80 ksi. Further, the yield strength is specified as 36 ksi. In some limited research, it appears as though the tensile strength variation is a result of temperature and strain rate, although I can't find anything definitive. To approve this substitution, I will need hard documentation that the specified rod will have a yield strength of 50 ksi and a tensile strength of 65 ksi at a minimum.

So my questions are as follows:

1) Why is A36 Grade 50 not listed in the AISC Manual? Is this material readily available?
2) Why the variation on tensile strength and is there a way to guarantee a minimum tensile strength of 65 ksi for A36?

Any input would be helpful!

 

[JStephen]

I've never heard of A36 Grade 50. The "36" is the grade, so that's like saying "Grade 36 Grade 50". A lot of the A36 products may run well over 36ksi yield, but there's no guarantees. A lot of the A36 products may also be dual/triple spec materials with SA36, A709, etc.
I was thinking A572 was specifically a plate specification, not sure if other shapes are produced in it.
If they're threaded, you might look into the spec for F1554 Gr. 55 anchor bolts.

 

[UBGrad]

Reference Table 2-3, 2-4, and 2-5 in AISC 13th edition before you specify a certain material. A572 Gr. 50 is a steel spec. for structural shapes, not threaded rods. Look into using F1554 Gr. 55 with Fy=55ksi and a tensile strength of 75-95ksi.

 

[BloEngineer]

I did a little more research on this and came to the same conclusion. I have never seen a Grade 50 option for A36, so not sure where the contractor got it from.

You're right that it's likely that the actual yield strength of A36 is higher than 36 ksi, but we must always assume the minimum because you just never know...

In digging a little deeper, I looked at ACI 350-06 Appendix D (this is a water treatment application) and found that Section D.6.1.2 allows the steel strength of anchors to be designed for 0.6 x Ase x fu "because typical anchor materials do not exhibit a well-defined yield point". There is a limitation on fu as a function of yield strength. So I assumed a 58 ksi tensile strength for A36 and rechecked the anchors in shear. They are coming out to be adequate for the slight reduction in tensile strength. Therefore, I plan on approving the use of ASTM A36 rods for this application.

 

[BloEngineer]

UBGrad, I did reference Table 2-5 in AISC 13th for the design. If you look at A572 Grade 50 for the application of threaded rods, it states that there is "other applicable material specification". I don't believe this means that the product doesn't exist, but I interpret it to mean that it just may not be common or easily obtained. A quick search online comes up with several steel fabricators that manufacture A572 Grade 50 rods. I'm assuming that since the contractor raised the issue that he is not used to seeing it spec'd and is concerned about the price/availability.

 

[MotorCity]

One slight detail to consider......if you specify F1554 Gr 36 rods, the contractor has the OPTION of supplying Gr 50 rods. So while the contractor may confuse A36 with F1554, he may really be saying that he is giving you Gr 50 instead of Gr 36 rods.

 

[MotorCity]

Correction........ Gr 55 rods (not Gr 50 as I stated above).

 

[BloEngineer]

MotorCity, I think confusion over ASTM grade is likely on the part of the contractor.

I found that A36 is acceptable, so I will probably stick with specifying it as the substitution. But, for future reference...what I require is a smooth rod that can be threaded. So one end is threaded (end that is epoxy anchored) and the "body" is smooth. I envision this being done in the shop where the fabricator will take a smooth rod and machine thread the end. Which brings me to the question of a rod versus a bolt/anchor bolt and what provides the flexibility I require. It seems that A36 is readily available as both threaded and smooth. Does F1554 have the same flexibility?

 

[MotorCity]

F1554 can be provided threaded or smooth. I believe that the difference in threading between A36 and F1554 is that one can be fully threaded and one cannot (I can't remember which is which off hand).

Why do you need the epoxied end to be threaded? If it is post installed you will not be able to install a nut on the end right?

 

[BloEngineer]

The threading is being provided as an extra measure of rigidity and to match tested/posted bond strength values (in this case, from Hilti). They have not tested smooth rod epoxy anchored to concrete.

 

[IFRs]

https://www.aisc.org/bookstore/itemRedirector.aspx?id=15296